composed of spindle-shaped fibers
diameter of 2-10 µm and lengths of several hundred µm
lack the coarse CT sheaths of skeletal muscle, but have fine endomysium
are typically organized into two layers (“longitudinal” and “circular”) of closely apposed fibers
found in walls of hollow organs (except the heart)
have essentially the same contractile mechanisms as skeletal muscle
microscopic anatomy
SR is less developed than in skeletal muscle and lacks a specific pattern
T tubules are absent
plasma membranes have pouchlike infoldings called caveoli
Ca2+ is sequestered in the extracellular space near the caveoli, allowing rapid influx when channels are opened
no visible striations and no sarcomeres
thin and thick filaments are present
organization of myofilaments
ratio of thick to thin filaments is much lower than in skeletal muscle
thick filaments have heads along their entire length
there is no troponin complex
thick and thin filaments are arranged diagonally, causing smooth muscle to contract in a corkscrew manner
noncontractile intermediate filament bundles attach to dense bodies (analogous to Z discs; composed of α-actinin) at regular intervals
no neuromuscular junctions
innervating nerves have bulbous swellings called varicosities
varicosities release neurotransmitters into wide synaptic clefts called diffuse junctions
visceral contractions
when the longitudinal layer contracts, the organ dilates and contracts
when the circular layer contracts, the organ elongates
peristalsis
alternating contractions and relaxations of smooth muscles that mix and squeeze substances through the lumen of hollow organs
whole sheets of smooth muscle exhibit slow, synchronized contraction
contract in unison, reflecting their electrical coupling with gap junctions
action potentials are transmitted from cell to cell
some smooth muscle cells:
act as pacemakers and set the contractile rate for whole sheets of muscle
are self-excitatory and depolarize without external stimuli
contractile mechanism
actin and myosin interact according to the sliding filament mechanism
final trigger for contractions is a rise in intracellular Ca2+
Ca2+ is released from the SR and from the extracellular space
Ca2+ interacts with calmodulin and myosin light chain kinase to activate myosin
role of Ca2+
binds to calmodulin and activates it
activated calmodulin activates the kinase enzyme
activated kinase transfers phosphate from ATP to myosin cross bridges
phosphorylated cross bridges interact with actin to produce shortening
muscle relaxes when intracellular Ca2+ levels drop
signal transductions mechanisms
phosphatidylinositol pathway
GS-protein-coupled pathway
nitric oxide (NO)-cGMP pathway
special features of SM contraction
unique characteristics:
smooth muscle tone
slow, prolonged contractile activity
low energy requirements
response to stretch
smooth muscle exhibits a phenomenon called stress-relaxation response in which:
responds to stretch only briefly, and then adapts to its new length
new length, however, retains its ability to contract
this enables organs such as the stomach and bladder to temporarily store contents
certain smooth muscles can divide and increase their numbers by undergoing hyperplasia
shown by estrogen’s effect on the uterus
at puberty, estrogen stimulates the synthesis of more smooth muscle, causing the uterus to grow to adult size
during pregnancy, estrogen stimulates uterine growth to accommodate the increasing size of the growing fetus
commonly found as visceral muscle, the cells of which:
contract rhythmically as a unit
are electrically coupled to one another via gap junctions
often exhibit spontaneous action potentials
are arranged in opposing sheets and exhibit stress-relaxation response
located in:
large airways to the lungs
large arteries
arrector pili muscles attached to hair follicles
internal eye muscles (sphincter & dilator pupillae mm)
characteristics:
rare gap junctions
infrequent spontaneous depolarizations
structurally independent muscle fibers
rich nerve supply, which, with a number of muscle fibers, forms motor units
graded contractions in response to neural stimuli
The illustrations for calcium-influx and signal transduction mechanisms are from
www.cvphysiology.com and used with permission.
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[ Anatomy & Physiology 1 syllabus ] [ Page created 04 September 2004 ][ Last update 01 November 2006 ] [ Questions about this lecture? E-mail me ] |